As a conventional heat pump apparatus, there is a heat pump type drying apparatus in which a heat pump is used as a heat source and drying air is circulated (see patent document 1 for example). FIG. 10 shows a structure of the conventional heat pump type drying apparatus described in the patent document 1.
In the clothing dryer shown in FIG. 10, a rotation drum 2 is used as a drying room which is provided in a body 1 of the clothing dryer so as to rotate freely. The rotation drum 2 is driven by a motor 3 through a drum belt 4. A blower 22 is driven by the motor 3 through a fan belt 8. The blower 22 sends drying air from the rotation drum 2 to a circulation duct 18 through a filter 11 and a rotation drum-side air intake 10.
The heat pump apparatus comprises an evaporator 23 which evaporates a refrigerant to dehumidify drying air, a condenser 24 for condensing the refrigerant to heat the drying air, a compressor 25 for generating a pressure difference in the refrigerant, an expansion mechanism 26 such as a capillary tube for maintaining the pressure difference of the refrigerant, and a pipe 27 through which the refrigerant passes. A portion of the drying air heated by the condenser 24 is discharged outside from the body 1 through an exhaust port 28.
Next, the operation of the drying apparatus will be explained. First, clothing 21 to be dried is placed in the rotation drum 2. Then, if the motor 3 is rotated, the rotation drum 2 and the blower 22 rotate and drying air flow B is generated. The drying air absorbs water from the clothing 21 in the rotation drum 2 and takes up much moisture and then, the air is sent to the evaporator 23 of the heat pump apparatus through the circulation duct 18 by the blower 22. The drying air from which heat is absorbed by the evaporator 23 is dehumidified and sent to the condenser 24 and heated therein, and the air is again circulated into the rotation drum 2. A drain outlet 19 is provide in a middle portion of the circulation duct 18, and a drain dehumidified and generated by the evaporator 23 is discharged out through the drain outlet 19. As a result, the clothing 21 is dried.
(Patent Document 1)
Japanese Patent Application Laid-open No. H7-178289
However, the structure of the conventional heat pump type drying apparatus has a problem that when the heat pump is operated under high temperature atmosphere, the discharge pressure of the compressor rises.
A principle of the discharge pressure rise of the compressor when the heat pump is operated under high temperature atmosphere will be explained. In a heat pump type drying apparatus having a circulation duct, input from an outside power source into the compressor and heat release from air circulating in the duct into outside become equal to each other in a steady state. That is, if the input into the compressor is constant, a difference between the atmosphere temperature and the average temperature of air in the circulation duct is always constant. Thus, if the atmosphere temperature rises, the average temperature of air in the circulation duct rises. For this reason, refrigerant pressure sucked into and discharged out from the circulation duct rises, and there is a danger that this pressure exceeds permissive pressure of the compressor.
The conventional structure has a problem that when the heat pump is operated under high temperature atmosphere, COP (coefficient of performance) of the heat pump is deteriorated, and electricity required for drying operation is increased.
A principle that the COP (coefficient of performance) of the heat pump when the heat pump is operated under the high temperature atmosphere is deteriorated will be explained. As described above, if the atmosphere temperature rises, the average temperature of air in the circulation duct rises, and a pressure of refrigerant sucked by the compressor rises. With this, the density of refrigerant sucked by the compressor is increased, and a circulation amount of the refrigerant in the heat pump cycle is increased. Thus, the heat pump cycle is shifted as shown in FIG. 11, an enthalpy difference of the refrigerant in the radiator is reduced, and the COP of the heat pump cycle is deteriorated.
The conventional structure has a problem that in the drying process, as the drying operation is proceeded, the drying speed is largely reduced, and the drying time is increased.
A reason why the drying speed is largely reduced as the drying operation is proceeded will be explained. Generally, when a solid body is to be dried using warm air, it is known that as the drying operation is proceeded, contend of water on a surface of the solid body to be dried is reduced, and the drying speed is reduced. In addition, when clothing is to be dried by using a rotation drum or the like, as the drying operation is proceeded, clothing is largely deviated in the rotation drum, and a transfer resistance of heat from the clothing surface to water remaining in the clothing is increased. Thus, according to the conventional structure, the transfer amount of heat into the clothing is reduced, the drying speed is further reduced as compared with general drying characteristics, and electric power consumption required for the drying operation is increased.
Further, an HFC refrigerant (a refrigerant including hydrogen atom, fluorine atom, and carbon atom in a molecule) which is currently used as a refrigerant of the heat pump apparatus directly affects the global warming and thus, it is proposed to convert such a refrigerant into a natural refrigerant such as carbon dioxide (CO2, hereinafter) existing in the natural environment as an alternative refrigerant. However, if the CO2 refrigerant is used, theoretic efficient of the heat pump system is low as compared with the HFC refrigerant, and the operating efficiency of the heat pump type drying apparatus is deteriorated. Thus, there is a problem that energy must be saved and the efficiency must be enhanced to reduce the indirect affect on the global warming by using a natural refrigerant such as CO2 which does not directly affect the global warming.
The present invention has been accomplished in view of the conventional problems, and it is an object of the invention to provide a drying apparatus which enhances its efficiency while avoiding the excessive rise of the discharge pressure of the compressor also under a high outside temperature condition when a refrigerant that is brought into a supercritical state on the radiation side of a heat pump cycle such as CO2 is used as a refrigerant.